Plastic materials used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together and form a smooth coating.
Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above-stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough finish referred to as an "orange peel" surface. The "orange peel" surface problem has caused thermosetting coatings to be applied from organic solvent systems which are inherently undesirable because of the environmental and safety problems that may be occasioned by the evaporation of the solvent system. Solvent-based coating compositions also suffer from the disadvantage of relatively poor percent utilization, i.e., in some modes of application, only 60 percent or less of the solvent-based coating composition being applied contacts the article or substrate being coated. Thus, a substantial portion of solvent-based coatings can be wasted since that portion which does not contact the article or substrate being coated obviously cannot be reclaimed.
Powder coatings that exhibit good weatherability are becoming increasingly important, particularly for outdoor applications. Commercial polyester resins for powder coatings are mostly based on terephthalic acid and isophthalic acid as diacids and neopentyl glycol as the diol with minor variations. Powder coatings based on these aromatic polyester resins provide good general properties but only fair weathering resistance.
Powder coatings based on acrylic resins are known to have excellent weathering performance but are generally more expensive.
British Patent No. 962,913 discloses polyesters containing cyclohexanedicarboxylic acid and 2,2,4,4-tetramethyl-1,3-cyclobutanediol residues useful as film and molding plastics.
U.S. Pat. No. 3,313,777 discloses polyesters containing cyclohexanedicarboxylic acid and 2,2,4,4-tetramethyl-1,3-cyclobutanediol residues useful as film and molding plastics.
U.S. Pat. No. 4,363,908 discloses copolyesters containing cyclohexanedicarboxylic acids and 2,2,4,4-tetramethyl-1,3-cyclobutanediol useful as bonding or coating compositions.
U.S. Pat. No. 4,525,504 discloses stabilized polyesters with improved weatherability based on 1,4cyclohexanedicarboxylic acid and 2,2,4,4-tetramethyl-1,3-cyclobutanediol. These polyesters are high molecular weight polyesters useful in molding plastics.
U.S. Pat. No. 4,910,292 discloses water-dissipatable polyesters useful in coatings. 2,2,4,4-Tetramethyl-1,3-cyclobutanediol and the cyclohexanedicarboxylic acids are listed as possible residues.
U.S. Pat. No. 5,097,006 discloses all aliphatic polyester resins based on 1,4-cyclohexanedicarboxylic acid and hydrogenated bisphenol A and other cycloaliphatic diols, useful as powder coating resins.